class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows==1)return s;
        string ret;
        int d=2*numRows-2,n=s.size();
        for(int i=0;i<n;i+=d)
        {
            ret+=s[i];
        }
        for(int k=1;k<numRows-1;k++)
        {
            for(int i=k,j=d-k;i<n||j<n;i+=d,j+=d)
            {
                if(i<n)ret+=s[i];
                if(j<n)ret+=s[j];
            }
        }
        for(int i=numRows-1;i<n;i+=d)ret+=s[i];
        return ret;
    }
};
